A: Birth control pills probably won’t, but life probably will.
TL;DR: Many people who use hormonal birth control (i.e., pills, patches, rings or shots) report gaining weight, but it just so happens that people who aren’t using birth control also gain weight, at the same rates! Modern life is fattening! In large reviews of data on thousands of people, use of hormonal birth control was NOT found to increase weight gain with one possible exception, the birth control shot (i.e., Depo Provera). No other method has been associated with weight gain. It’s important to know the facts, as fear of weight gain often prevents people who need birth control from using it.
Most people gain weight over time. 
This is very true during the teen years, as they’re still growing! On average, girls gain 20 pounds from age 14 to 20. While many people start birth control during this time frame, studies show that teenage pill, patch, ring, IUD, and implant users do not gain any more weight than non-users (in fact, they often gain less). Adults also tend to gain weight, about 3-10 pounds per decade in general, but users of the above birth control methods do not gain more weight on average than non-users.
The birth control shot is a higher dose of hormone than other methods. Studies of the shot show that about 1 in 4 users will gain an extra 5 pounds (2.3 kg) per year on average compared with non-users. If this weight gain does occur, it is obvious in the first 6 months of use, and the person can quickly switch to a different method. About 3 in 4 shot users do not gain extra weight.
The vast majority of people do not gain weight from their hormonal birth control, as evidenced by large data sets. That said, every body is unique, so it is possible that every now and then an individual could truly gain weight due to their hormonal method. But it must be infrequent, as the effect can’t be seen in large studies. Weight gain is common in general. Birth control is rarely to blame.
Many people avoid using hormonal birth control due to fear of side effects such as weight gain. This can be a problem, as almost half of all pregnancies in the United States are UNINTENDED. 
Often in these cases, no birth control was used. Side effects can be real, but it’s important to know that some are more myth than reality.
We can be confident that most hormonal birth control methods do not cause weight gain in the vast majority of people. People just tend to gain weight, whether on birth control or not. Those who are at risk of pregnancy should not let fears of weight gain prevent them from using effective birth control. Talk with your clinician about what’s right for you!
Stay safe, stay healthy!
Love,
Those Nerdy Girls
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Additional Resources
Brief synopsis of studies on birth control and weight gain
Large review of data on birth control pills and weight by Cochrane Library
Large review of data on progesterone-only birth control and weight by Cochrane Library
Potential weight gain from Depo-Provera will be evident in first six months
